Welcome to NASIOC - The world's largest online community for Subaru enthusiasts!

Welcome to the NASIOC.com Subaru forum.

You are currently viewing our forum as a guest, which gives you limited access to view most discussions and access our other features. By joining our community, free of charge, you will have access to post topics, communicate privately with other members (PM), respond to polls, upload content and access many other special features. Registration is free, fast and simple, so please join our community today!

If you have any problems with the registration process or your account login, please contact us.

Has anyone lowered their compression using a thicker head gasket? If so how much HP are you running? What thickness did you use? How many miles do you have with the mod?

I'm working on a turbo SVX project. I hear that changing the pistons is a better way to go, but if people are having success with this mod on their cars I think I may do this to save some $$$$. I am considering lowering the compression from 10.1:1 to 8.5:1.

EJ22t rods/pistons are a straight drop in for your motor. As for the compression thing... there are many engine math resources out there to see how head gasket thickness effects compression.

Thicker/Thinner head gaskets are more prone to blowing unless they're something fairly exotic. Fuji engineers aren't stupid and spent a great deal of time determining how thick the head gasket needed to be to cope with normal expansion/warping of the head/block mating surface during normal operation.

That is some good info on the effects of a thick headgasket. In the MY00 EJ25 it goes from 11.2:1 to 10.1:1 (I question the first numbers accuracy), and in a Legacy EJ22t from 8.0:1 to 7.2:1.

So roughtly .6-.8 compression differance is what I would say. I don't know what SVX's stock compression is but if you are not under 9.0:1 (or close) I would say that is risky to run anything above 8psi.

Of course a large intercooler, and water/alcohol injection can offset some detonoation.

That is some good info on the effects of a thick headgasket. In the MY00 EJ25 it goes from 11.2:1 to 10.1:1 (I question the first numbers accuracy), and in a Legacy EJ22t from 8.0:1 to 7.2:1.

So roughtly .6-.8 compression differance is what I would say. I don't know what SVX's stock compression is but if you are not under 9.0:1 (or close) I would say that is risky to run anything above 8psi.

Of course a large intercooler, and water/alcohol injection can offset some detonoation.

Stock MY00 is "factory rated" at 10.1:1 - Wayne actually measured volume and ran the calcs. True compression ratio turns out to be a lot higher than the factory states, as he notes on his site.

FWIW, a thicker headgasket will really mess with your valve timing. By spacing the heads out, it throws off the stock timing rather quickly.

The really bad part is, the geometry of the timing belt means that for every degree of timing change you get on the passenger side head, you get 3 degrees on the driver side.

Back when I replaced the head gaskets on my '99 engine, I thought about using '98 gaskets instead. The reduction in compression ratio was very nice. But when I did some calculations on the timing change, it was pretty scary. So I realized it was a VERY bad idea.

I also find it odd that he lists the 99 EJ25 as 10:1 (not in the calculated section), but it was advertised as 9.7:1 (the 99 and 2000 models both have the same SOHC engine, but use different piston designs).

The calculated CR differing from advertised CR is definitely interesting, though. Could this possibly explain why a supposed increase of only 0.3 on the CR (9.7:1 to 10:1), led to the 2000+ engines taking boost so much more poorly than the 99 engines? Too bad the calculated field is blank on the MY99 line.

What about the 98s? When i got my car, the company tore down the engine and cleaned out all the carbon, replaced all the seals and gaskets. And when he showed me the Stock Head Gaskets to some thinner ones, i decided on the thinner gasket. If i can find out the two thicknesses, can i figure out my total compression?

In addition ot a higher compression ratio, your cam timing is now somewhat screwed up, just like I mentioned in an earlier post. Again, the biggest problem is not the change in timing, it's that the timing change is different from one side of the engine to the other.